1. Field of the Invention
This invention relates generally to cardiac stimulator leads, and more particularly to a cardiac stimulator lead having an improved structure for joining the ends of two conductor wires.
2. Description of the Related Art
Conventional cardiac stimulator systems consist of a cardiac stimulator and an elongated flexible cardiac lead that is connected proximally to a header structure on the cardiac stimulator and is implanted distally at one or more sites within the heart requiring cardiac stimulation or sensing. The cardiac stimulator is normally a pacemaker, a cardioverter/defibrillator, a sensing instrument, or some combination of these devices. At the time of implantation, the distal end of a cardiac lead is inserted through an incision in the chest and manipulated by the physician to the site requiring electrical stimulation with the aid of a flexible stylet that is removed prior to closure. At the site requiring electrical stimulation, the distal end of the lead is anchored to the endocardium by an active mechanism, such as a screw-in electrode tip, or alternatively, by a passive mechanism, such as one or more radially spaced tines that engage the endocardium. The proximal end of the lead is then connected to the cardiac stimulator and the incision is closed. The implantation route and site are usually imaged in real time by fluoroscopy to confirm proper manipulation and placement of the lead.
A conventional cardiac stimulator lead normally consists of an elongated, flexible, tubular, electrically insulating sleeve that is connected proximally to a connector that is adapted to couple to the header of a cardiac stimulator. In pacing leads, the distal end of the insulating sleeve is joined with a tip electrode. In defibrillator leads, a defibrillator or shock coil commonly projects from the distal end of the insulating sleeve. The shock coil consists of an uninsulated coiled wire wound with a large number of coils. The plurality of coils distribute defibrillation pulses over a much larger surface area of the myocardium than a pacing electrode.
In some conventional defibrillator lead designs, the electrical pathway between the lead connector and the shock coil is provided by a separate conductor wire that is coupled proximally to the connector and secured distally to a crimp assembly. The conventional crimp assembly consists of an inner tubular sleeve over which respective ends of the shock coil and the other conductor wire are positioned and crimped into position by respective outer crimp sleeves. The inner sleeve and the outer sleeves are normally made of titanium or other relatively rigid biocompatible conducting materials. The inner tubular sleeve is of such length that the ends of the shock coil and the other wire are usually not intertwined. The conducting nature of the inner sleeve is relied upon to pass current between the two wires.
A conventional crimp assembly can significantly hamper the movement of a stylet used to spatially manipulate the lead during implantation. For most implantation procedures, the physician inserts a stylet into the lead connector and advances it to the distal tip of the lead. The physician then manipulates the stylet to accurately position the distal end of the lead proximate the endocardial site requiring electrical stimulation. The distal end of the stylet must be inserted through the crimp assembly in order to reach the tip of the lead. This step may not be problematic where the stylet is not bent significantly prior to insertion, as is often the case where the implantation involves a relatively straight pathway through the heart. Fixation to the right ventricular apex is an example of such a relatively straight pathway.
Where the implantation requires the pathway of the lead tip to be deviated, the situation may become more difficult for the physician. For example, fixation to the superior interventricular septum or access to the great cardiac vein via the coronary sinus require the lead tip to be turned abruptly after entry into the heart. This is frequently accomplished by introducing a severe bend in the distal end of the lead, usually after the lead is initially positioned inside the heart. Initially, a straight stylet is used to move the lead into the right atrium. Then the straight stylet is removed and a highly curved stylet is inserted and advanced to the distal end of the lead. The stylet is usually curved by the physician by hand based on the physician's experience and knowledge of the patient's particular anatomy. The radius of curvature of the bend may be quite small.
The initial movement of the highly curved portion of the stylet through the lead may be unremarkable since the majority of the lead is quite flexible. As the curved portion is advanced, the lead is able to temporarily conform to the curvature of the stylet. In contrast to the insulating sleeve, the crimp assembly is quite rigid and cannot conform to the curvature of the stylet. As a result, the physician may encounter significant resistance to further axial movement when the highly curved portion of the stylet encounters the inner sleeve. This undesirable tactile response is more than just a nuisance. The natural tendency of the physician at this point is to apply additional thrust to the proximal end of the stylet to force the curved portion through the inner sleeve. Because the stylet is highly curved and thrust is being applied at the opposite end thereof, the stylet will tend to behave like an unstable column under compression loading. If the rubbing of the inner sleeve is great enough, axial thrust applied by the physician will cause the stylet to buckle and plastically deform at one or more points along its length. With one or more unintended bends in the stylet, the movement of the lead in response to manipulation of the stylet may be unpredictable and the complexity of the implantation procedure increased.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.